Conventionally, there is a known structure in which an electrolyte solution filling opening from which electrolyte solution is to be filled into a container is provided to a container in an energy storage device including an electrode assembly and the container for housing the electrode assembly. For example, Patent Document 1 discloses a battery including a battery container for housing a power generating element, a lid body for sealing an opening portion of the battery container, and an electrolyte solution filling opening which is formed in the lid body and from which electrolyte solution is to be filled. In this battery, a positive electrode lead of the power generating element is welded to the container and the electrolyte solution filling opening provided to the container is sealed with a positive electrode terminal. In other words, the positive electrode terminal also performs a function of sealing the electrolyte solution filling opening.